Making sure you and your family are vaccinated each year is the safest way to protect them against the flu. But sometimes, people might get flu symptoms even though they have been vaccinated against the flu.
People can get sick from other respiratory viruses such as rhinoviruses (which are associated with the common cold) and cause symptoms similar to the flu and also spread during flu season. The flu vaccine only protects against influenza, not other illnesses.
Another explanation is that it is possible to be exposed to influenza viruses, which cause the flu, shortly before getting vaccinated or during the two-week period after vaccination when the body is still developing immune protection.
Lastly, people who have been vaccinated may still get the flu if they are exposed to a flu virus that is very different from the viruses the vaccine is designed to protect against. There are many different flu viruses that spread and cause illness among people. For more information, see the Centers for Disease Control and Prevention’s information on different flu viruses.
Sources:
CDC – Misconceptions about Seasonal Flu and Flu Vaccines
CDC – Influenza (Flu) Viruses